Why do some wooden surfaces absolutely refuse poly clear coat even after having been sanded & stained?

For example, an old desktop (oak), completely sanded and wiped with mineral spirits. After applying stain, then letting it dry, the clear coat (Polyurethane) appears to have small ';craters'; on the surface. Re-applying another coat after the first one is dry and sanded shows the same problem.Why do some wooden surfaces absolutely refuse poly clear coat even after having been sanded %26amp; stained?
Some clear coatings are not compatible with each other. To avoid this problem apply a wax free shellac as a bonding coat. Zinsser seal coat is one good option. Parks (a Zinsser subsidiary) sells the same product with their label. After that you can finish it with any clear finish.Why do some wooden surfaces absolutely refuse poly clear coat even after having been sanded %26amp; stained?
It's from oil within the pores of the wood. I recommend applying a clear sealer such as shellac first.





Also be sure to let the mineral spirits eveaporate for several hours, perhaps overnight.
make up of the wood, most likely from oil